This invention relates to a brush assembly for a miniature electric motor, and to a method of forming the same.
The use of a graphite or graphite-based brush head mounted on a brush arm for use in an electric motor is known. Graphite segment commutators generally use this type of brush. The graphite on graphite interface significantly reduces wear and thus dust. It also generates less electrical noise.
The reduction in wear means that only a very short brush head is required allowing material savings and better space utilization within the motor. However, attachment of a shorter brush to the brush arm proves to be problematic using standard techniques due to the lack of a sizeable brush body to hold. Direct soldering is always difficult and requires a brush body with a high concentration of copper. Due to health reasons, soldering is generally avoided where possible. The common technique of using flaps on the brush arm which are resiliently deformed to grip a portion of the brush requires a significant root portion for the attachment. Hence, there is a need for a secure and reliable engagement of a brush head to a brush arm which overcomes the above mentioned problems.
According to a first aspect of the present invention, there is provided a brush assembly for a miniature electric motor comprising: a brush arm comprising an elongate strip of resilient conductive material having a distal end and a proximal end, the distal end having a cut out portion; and a brush head of graphite material having a projection residing in the cut out portion and a cap on the projection, the cap securing the brush head to the brush arm.
Preferably, the cut outs are apertures and the brush head has projections passing through the apertures to form one or more caps on the opposite side of the brush arm.
Preferably, the graphite material includes a low temperature thermosetting binder.
Preferably, the binder is cured by a hot pressing process which is used to form the caps.
According to a second aspect, the present invention provides a method of forming a brush assembly for a miniature electric motor, the method comprising the steps of: placing a preformed brush arm having at least one cut out portion in a mould; introducing brush material into the mould; pressing the brush material to form a brush head attached to the brush arm; and pressing the brush material into the at least one cut out portion forming an anchorage integral with the brush head thereby holding the brush head in intimate contact with the brush arm.
Preferably the brush material is substantially graphite mixed with a thermoset resin binder (such as phenolic) which is cured by using a hot pressing process to attach the brush head to the brush arm.
Preferably, the green brush material is introduced into the press die as a billet or as a preformed green brush head.